Field of the Invention
The present invention relates to a method and an apparatus for magnetic resonance imaging, and in particular, to a method and apparatus for magnetic resonance imaging by use of the Look-Locker method.
Description of the Prior Art
Magnetic resonance imaging is a bio-magnetic nuclear spin imaging technology developed rapidly along with the development of computer technology, electronic circuit technology, and superconductor technology. In magnetic resonance imaging, human tissue is placed in a static magnetic field B0, and then hydrogen nuclei within the human tissue are excited by a radio-frequency pulse with the same frequency as the precession frequency of the hydrogen nuclei, so as to cause resonance of the hydrogen nuclei in the tissue. The excited nuclei absorb energy and after the radio-frequency pulse ceases, the excited hydrogen nuclei emit a radio signal at a specific frequency and release the absorbed energy. This energy is detected by a receiver, and be processed by a computer to obtain an image.
In the Look-Locker method for longitudinal relaxation time imaging (T1 Mapping), a delay time (tf time) is needed between an inverse recovery pulse and an image acquisition sequence (e.g. FLASH sequence or other various sequences) in order to meet certain scanning requirements, such as a necessary spoiler gradient, a flexible inversion time (TI), and elimination of the magnetic transfer effect, and so on. In this case, the magnetization quantity proceeds through two relaxation processes: a relaxation process of a longitudinal relaxation time (T1) in the delay time, and a relaxation process of a longitudinal relaxation time (T1*) modulated by an image acquisition sequence in the image acquisition process. However, the Look-Locker method in the prior art does not take into account the relaxation process of the longitudinal relaxation time (T1).
Specifically, the Look-Locker method in the prior art is a method for measuring a longitudinal relaxation time. Immediately following the inverse recovery pulse and the delay time, longitudinal magnetization is extracted by the image acquisition sequence (e.g. a series of accelerated scramble phase gradient echo pulse sequences (Turbo FLASH Sequence)), so as to acquire a series of complete images with different inverse time in the magnetization quantity recovery process.
Because the recovery of the magnetization quantity is modulated by a radio frequency sequence of the FLASH sequence, the modulated relaxation time T1* is taken as the characterization quantity for relaxation in the process of gradually tending to be in a steady state. The relaxation process is as represented in the following equation [1].M(t)=M0*−(M0+M0*)exp(−t/T1*)  [1]wherein M0* is the steady-state magnetization quantity modulated by the image acquisition sequence, and M0*=M0·T1*/T1.
With regard to the longitudinal relaxation time (T1), three parameters (M0, M0* and T1*) of an image may be fitted in the unit of pixel according to equation [1], where M0 denotes an initial magnetization quantity, T1* denotes a modulated relaxation time, and M0* denotes a steady-state magnetization quantity modulated by the image acquisition sequence. Therefore, the longitudinal relaxation time (T1) is obtained directly from the fitted parameters, i.e. equation [2]:T1=T1*M0/M0*;  [2]